III-v compounds such as GaN, AlN, AlGaN, and InAlGaN have unique physical and electronic properties that make them ideal candidates for a variety of electronic and opto-electronic devices. In particular, these materials exhibit a direct band gap structure, high electric field breakdown, and high thermal conductivity. Additionally, materials such as InxAl1−xGaN can be used to cover a wide range of band gap energies, i.e., from 1.9 eV (where x equals 1) to 6.2 eV(where x equals 0). Unfortunately, although the III-V compounds are attractive for semiconductor applications due to their physical and electronic properties, until recently the development of devices based on III-V compounds has been limited by the lack of material with suitable conductivity, specifically p-type material.
In the late 1980's, p-type GaN was grown, followed rapidly by the development of fabrication techniques for p-type AlGaN material. These materials were grown using metal organic chemical vapor deposition (MOCVD) techniques and, to a lesser extent, using molecular beam epitaxy (MBE) techniques. Since the development of p-type III-V material, a variety of semiconductor devices employing both p-n and p-i-n junctions have been demonstrated, including light emitting diodes (LEDs), laser diodes, and photo-detectors.
In the MOCVD technique, III-V compounds are grown from the vapor phase using metal organic gases as sources of the Group III metals. For example, typically trimethylaluminum (TMA) is used as the aluminum source and trimethylgallium (TMG) is used as the gallium source. Ammonia is usually used as the nitrogen source. In order to control the electrical conductivity of the grown material, electrically active impurities are introduced into the reaction chamber during material growth. Undoped III-V compounds normally exhibit n-type conductivity, the value of the n-type conductivity being controlled by the introduction of a silicon impurity in the form of silane gas into the reaction chamber during growth. In order to obtain p-type material using this technique, a magnesium impurity in the form of biscyclopentadienylmagnesium is introduced into the reactor chamber during the growth cycle. As Mg doped material grown by MOCVD is highly resistive, a high temperature post-growth anneal in a nitrogen atmosphere is required in order to activate the p-type conductivity.
Although the MOCVD technique has proven adequate for a variety of commercial devices, it has a number of limitations that constrain the usefulness of this approach. First, due to the complexity of the various sources (e.g., trimethylaluminum, trimethylgallium, and biscyclopentadienylmagnesium), the process can be very expensive and one which requires relatively complex equipment. Second, the MOCVD technique does not provide for a growth rate of greater than a few microns per hour, thus requiring long growth runs. The slow growth rate is especially problematic for device structures that require thick layers such as high voltage rectifier diodes that often have a base region thickness of approximately 30 microns. Third, n-type AlGaN layers grown by MOCVD are insulating if the concentration of AlN is high (>50 mol. %). Accordingly, the concentration of AlN in the III-V compound layers forming the p-n junction is limited. Fourth, in order to grow a high-quality III-V compound material on a substrate, the MOCVD technique typically requires the growth of a low temperature buffer layer in-between the substrate and III-V compound layer. Fifth, generally in order to obtain p-type III-V material using MOCVD techniques, a post-growth annealing step is required.
Hydride vapor phase epitaxy or HVPE is another technique that has been investigated for use in the fabrication of III-V compound materials. This technique offers advantages in growth rate, simplicity and cost as well as the ability to grow a III-V compound layer directly onto a substrate without the inclusion of a low temperature buffer layer. In this technique III-V compounds are epitaxially grown on heated substrates. The metals comprising the III-V layers are transported as gaseous metal halides to the reaction zone of the HVPE reactor. Accordingly, gallium and aluminum metals are used as source materials. Due to the high growth rates associated with this technique (i.e., up to 100 microns per hour), thick III-V compound layers can be grown.
The HVPE method is convenient for mass production of semiconductor devices due to its low cost, flexibility of growth conditions, and good reproducibility. Recently, significant progress has been achieved in HVPE growth of III-V compound semiconductor materials. AlGaN and AlN layers have been grown as well as AlGaN/GaN heterostructures using this technique. The AlGaN alloys grown by HVPE have been found to be electrically conductive up to 70 mol. % of AlN. Furthermore, since this technique does not require low temperature buffer layers, diodes with n-GaN/p-SiC heterojunctions have been fabricated with HVPE.
In order to fully utilize HVPE in the development and fabrication of III-V compound semiconductor devices, p-type layers must be produced. Conventional HVPE techniques have been unable, however, to grow such material. For example, if a magnesium acceptor impurity is added to a III-V layer grown utilizing conventional HVPE techniques, the resultant material is insulating (i.e., i-type) rather than being p-type. As a result, the potential of the HVPE technique for fabricating p-n or p-i-n junction devices has not been realized.